go/packages: disable -mod=readonly behavior in overlays

This was preventing mod files from being updated when dependencies were
added to overlays. This prevents gopls from accurately modifying a user's
mod file when a dependency is added, which poses problems for usability.
To handle the issues that this was (supposedly) solving, we can require
that files from multiple modules are never added in an overlay.

For some reason, this doesn't break TestOverlayModFileChanges.

Change-Id: I24d6c30754c2bebc440802f7cf2931d98e450ebf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/211058
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

go/packages/golist.go

@@ -253,12 +253,7 @@ func addNeededOverlayPackages(cfg *Config, driver driver, response *responseDedu
 	if len(pkgs) == 0 {
 		return nil
 	}
-	drivercfg := *cfg
-	if getGoInfo().env.modulesOn {
-		drivercfg.BuildFlags = append(drivercfg.BuildFlags, "-mod=readonly")
-	}
-	dr, err := driver(&drivercfg, pkgs...)
-
+	dr, err := driver(cfg, pkgs...)
 	if err != nil {
 		return err
 	}